The present invention relates to a co-current cyclone separator. This equipment, used in chemical engineering, is an apparatus which makes it possible to separate a dense phase D1 contained in a mixture M1 containing the said dense phase D1 and a light phase L1.
The present invention likewise relates to the use of this improved cyclone separator for the rapid separation of a dense phase D1 and a dilute phase L1 from their mixture M1.
According to the prior art, several types of cyclone are already known the performance levels of which are usually evaluated on a basis of their efficiency in collecting the dense phase D1 and the loss of head of the light phase L1 in the cyclone separator (hereinafter referred to as the apparatus). In the vast majority of cases, apparatuses of this type are designed with an eye to achieving the greatest possible efficiency in collecting the dense phase D1 while limiting as far as possible the loss of head of the light phase L1.
A first type of cyclone is the reverse flow cyclone in which the mixture M1 containing the phases D1 and L1 tangentially enters the enclosure of the cyclone in the immediate vicinity of its top which, at least for the light phase L1, induces a vortex and the centrifugal force which is derived therefrom makes it possible for the dense phase D1 to migrate to the wall of the enclosure where it progresses in a spiral (following a helical movement) towards the bottom of the separator where it is normally collected or evacuated via a collector cone at the level of which the vortex of the light phase is reversed. The light phase L1 having changed direction emerges in counter-current from the dense phase D1 to the end of the separator where the intake of mixture M1 is located.
A second type of cyclone is the co-current cyclone in which the mixture M1 containing the phases D1 and L1 enters axially or tangentially. In the case of an axial intake, the vortex is normally initiated with the help of blades in the form of a helix. In this type of cyclone, the outlet of the light phase L1 and the outlet of the dense phase D1 are situated close to the same end of the cyclone which is the end opposite that through which the mixture M1 is introduced into the apparatus. Therefore, there will be an outlet referred to as an internal or interior outlet through which the light phase L1 is discharged and an outlet referred to as an external or exterior outlet through which the dense phase D1 is discharged.
For certain applications, such as for example in the case of the process referred to as ultrapyrolysis, described for example by Graham et al, World Fluidisation Conference, May 1986, Elsinore Denmark, which is a high temperature cracking process performed in a fluidised state and with gas dwell times in the reactor of less than one second, it is necessary to use a very high speed separator. In this process, the chemical reaction of thermal cracking is initiated by heat-bearing solids and occurs in a piston flow reactor. The reaction time is very short, usually about 100 to about 900 milliseconds (ms) and it is important, if the process is to be properly efficient thermally, very rapidly to separate the solids from the gases before carrying out any rapid hardening of the gaseous products. The dwell time in the separator should be as short as possible and furthermore the distribution of dwell times must be as narrow as possible in order to minimise secondary cracking reactions which might result in the deterioration of exploitable products.
By reason of its very principle, based on the turn-round of the gaseous phase, it is scarcely possible to alter the geometry of a return flow cyclone in order to limit the dwell time of the light phase L1 in the apparatus. The length (Lc) of the apparatus is indeed imposed by the natural length of the vortex (Lv) as is for example described by R. M. Alexander in Fundamentals of cyclone design and operation, Proc. Aus. I.M.M., 1949, pages 203-228, or by S. Bryant et al, hydrocarbon processing, 1983, pages 87-90. This length (Lv) is usually around 3 to 4 times the diameter (Dc) of the apparatus. If the length of the apparatus is reduced then the vortex will bear on the outlet cone of the dense phase D1 causing a re-entrainment of the light phase by the dense phase circulating in a spiral towards its outlet. If one increases the speed of intake of the mixture M1, there is a simultaneous increase in erosion at the level of the tangential intake which is not industrially desirable.
In a co-current cyclone, the dense and light phases circulate in the same direction. The dense phase is drawn off through an external pipe and the light phase through an internal pipe, of which the entrance, referred to as the internal inlet, is situated at a distance (Ls) which may be much shorter than the length (Lc) of the reverse flow cyclone. This internal intake can be quite close to the intake of the mixture M1 but the closer it is the more the light phase will tend to circulate into the external outlet, around the internal pipe, before emerging again under the influence of the helical movement of the phases which make up the mixture. Furthermore, the closer the internal intake is to the intake for the mixture M1, the more collection of the dense phase D1 will be subject to the influence of turbulences existing at the level of the mixture intake. For example, in the case of a conventional `flat roof` tangential intake, the flow of phases into the entrance is altered by interference and turbulence which throw part of the dense phase into the central part of the apparatus producing a reduction in efficiency of the collection of the dense phase D1 which will be all the more substantial the closer the internal intake of light phase L1 is to the tangential intake of the mixture M1.
In this type of co-current, in contrast to what happens with reverse flow cyclones, it is possible by placing the internal intake of the light phase fairly close to the intake of the mixture M1 (at a distance which is less than the length (Lc) of the reverse flow cyclone, and monitoring the circulation of the light phase in the internal intake and the flow in the intake of mixture M1, to obtain a rapid separation of the phases while retaining a satisfactory level of efficiency of collection of the dense phase D1 and while enjoying an acceptable distribution of dwell time of the light phase.